The invention relates to a wind turbine and a method for operating a wind turbine. The wind turbine comprises a transformer. In the method, electrical energy is produced by means of a generator and fed into an electrical grid system in a number of phases. Electrical energy is fed to the secondary side of the transformer at a low voltage. On the primary side of the transformer, the electrical energy is output at a higher voltage. The potential on the primary side of the transformer in relation to the ground potential is undefined.
Generators in wind turbines produce electrical energy at a low voltage of e.g. 690 volts. By means of a transformer associated with the wind turbine, the electrical energy is stepped up to a medium voltage of, e.g., 10 kV, 20 kV or 30 kV, and is transmitted to a medium-voltage system. Via the medium-voltage system, the electrical energy is delivered to a transformer substation of a system operator where, for the purposes of further transmission, it is stepped up to an even higher voltage of, e.g., 400 kV (or else 110 kV, 132 kV, 220 kV or 380 kV). The greater the size of the wind farm concerned, and the more remote its site of construction (e.g., in the case of offshore wind farms), the longer the associated spans for the transmission of electrical energy in the medium-voltage system will be.
On the primary side of the transformer, the potential in relation to ground is undefined. Accordingly, the primary side itself is not grounded, or in any event only weakly grounded or grounded with a high resistance, and any grounding point in the medium-voltage system is so remote that, as a result of capacitances, inductances and resistances on the lines, the potential on the primary side of the transformer may deviate from the potential at the grounding point.
Consequently, in case of a fault on the medium-voltage system (e.g., resulting from a ground fault), information on the fault concerned may not reach the wind turbine immediately. Accordingly, the wind turbine would continue the in-feed of electrical energy to its full capacity, even though the medium-voltage system possibly no longer has any capability whatsoever of outputting the electrical energy again at the other end. In this case, the cables in the medium-voltage system would sustain damage within a very short space of time. Repairs to cables in the medium voltage system are both elaborate and expensive.